The present invention relates to a molded case circuit breaker, and more specifically, to an assembled structure of a current-interrupting section having fixed contact shoes and a rotary bridge-type contact shoe.
First, a general configuration of a molded case circuit breaker is schematically shown in FIG. 5. In this figure, reference numeral 1 denotes a main body case of a circuit breaker, 2 is an opening and closing handle or a switching handle, 3 is a toggle type opening and closing mechanism or a switching mechanism, and 4 is an over-current tripping device (based on a bimetal method or the like). Reference numeral 5 is a current-interrupting section containing fixed contact shoes, a movable contact shoe and arc extinguishing devices of a main circuit, and is linked to the opening and closing mechanism 3. An opening and closing operation of such a circuit breaker is well known. Through an ON/OFF operation of the opening and closing handle 2, the movable contact shoe in the current-interrupting section 5 opens or closes via the opening and closing mechanism 3. Further, when an over-current flows to activate the over-current tripping device 4 while the main circuit is powered, the opening and closing mechanism 3 performs a trip operation to open the movable contact shoe in the current-interrupting section 5, thereby interrupting the over-current flowing through the main circuit.
An interrupting method used for the current-interrupting section 5 includes a single-break method and a double-break method. An example of a double-break method using a movable contact shoe equipped with a bridging rotary contact shoe is disclosed in Japanese Patent Publications (KOKAI) No. 06-028964 and No. 06-52777. A configuration of this circuit breaker is shown in FIGS. 6(a) and 6(b). In this figure, reference numeral 6 denotes an insulated case of the current-interrupting section 5; 7 shows power-supply-side and load-side fixed contact shoes disposed in the insulated case 6 and arranged diagonally opposite to each other; 7a is a fixed contact provided at a tip portion of each fixed contact shoe; 8 is a movable contact shoe that bridges the contacts of the fixed contact shoes 7; 9 is a rotary drum-shaped contact shoe holder that holds the movable contact shoe 8; and 10 is an arc extinguishing device or a grid positioned at each side of the movable contact shoe 8 and disposed in the insulated case 6. The movable contact shoe 8 is loosely fitted in a through-hole 9a formed in the contact shoe holder 9, and is urged and held in position in the contact shoe holder 9 by a pressure spring (a tension spring or a torsion coil spring) 11.
In the illustrated structure, four tension springs are provided as the pressure spring 11, and two of them are arranged at each of the right and left sides of the movable contact shoe 8. Each tension spring is disposed vertically relative to the movable contact shoe 8, and extends between a rod 12 supported between right and left side walls of the contact shoe holder 9 and a rod 13 disposed on a top or bottom surface of the movable contact shoe 8. In this state, the movable contact shoe 8 is positioned and held in a floating sate and is pressed so that a force acts counterclockwise relative to the center of rotation xe2x80x98Oxe2x80x99 thereof. Thus, at an activation position shown in the figure, a predetermined contact pressure is exerted between the movable contact shoe 8 and the contact 7a of each of the fixed contact shoes 7.
Further, the tip portion of each of the fixed contact shoes 7 is folded in a U-shape. When an over-current such as short circuit current flows through the main circuit, the movable contact shoe 8 is substantially instantaneously opened before the over-current tripping device 4 (see FIG. 5) operates by an electromagnetic resilient force exerted between the tip portions of the fixed contact shoes 7 and the movable contact shoe 8. Furthermore, the folded portion of each fixed contact shoe has a magnetic yoke 14 to enhance a magnetic field acting on an arc generated between the contacts of the fixed and movable contact shoes during current interruption, thereby increasing the electromagnetic arc driving force to the arc extinguishing devices 10.
A configuration in which the pressure spring 11 is formed of torsion coil springs instead of tension springs has been disclosed and known in Japanese Patent Publication (KOKAI) No. 01-166429. This assembled structure is shown in FIGS. 7(a) and 7(b). In this configuration, the pressure spring 11 is formed of two torsion coil springs, each being disposed at the right or left side of the movable contact shoe 8. One of the springs has its opposing ends interposed between a recess groove 8a formed on a top surface side of the movable contact shoe 8 and the contact shoe holder 9, while the other spring has its opposing ends interposed between a recess groove formed on a bottom surface side of the movable contact shoe 8 and the contact shoe holder 9. The springs thus urge the movable contact shoe 8 counterclockwise with a predetermined contact pressure between the movable contact shoe 8 and the contact 7a of each of the fixed contact shoes 7 like the case shown in FIG. 6.
The current-interrupting section described above has the following problems. Namely, in the structure in FIG. 6(b) in which the pressure spring 11 of the movable contact shoe 8 is formed of the tension springs, the rods 13 and the rods 12 are provided at the top and bottom surfaces of the movable contact shoe piece 8 between the right and left side walls of the contact shoe holder 9, respectively. And, each tension spring is extended between the corresponding rods 13 and 12. The resulting structure becomes complex, and the procedure for mounting the pressure springs 11 is cumbersome. Furthermore, due to the nature of the tension springs, when the movable contact shoe 8 is opened by the electromagnetic force resulting from over-current, the spring force in a return direction, which pushes back the movable contact shoe piece 8 to a closed position, increases. Accordingly, in order to prevent inadvertent return contact between the contacts due to this spring force, a special mechanism for latching the movable contact shoe 8 to its open position is required, further complicating the structure and assembly.
In the structure in FIGS. 7(a) and 7(b), the pressure spring 11 is formed of the two pairs of the torsion coil springs arranged at the right and left sides of the movable contact shoe 8, and each spring is interposed in the space between the movable contact shoe 8 and the right or left side wall of the contact shoe holder 9. However, the space available between the movable contact shoe piece 8 and the right or left side wall of the contact shoe holder 9 is limited so small that if the torsion coil spring interposed in the corresponding space has a small coil length, it needs to have a large spring constant in order to exert a predetermined contact pressure on the movable contact shoe 8. Thus, even if the pressure spring 11 comprises the torsion coil springs as shown in FIGS. 7(a) and 7(b), when the movable contact shoe 8 is opened by the electromagnetic reaction force resulting from the over-current as described above, the spring forces of the torsion coil springs cause the delay of the opening. Moreover, when extension of an arc generated between the contacts suppresses the current and reduces the electromagnetic reaction force exerted on the movable contact shoe, the spring force pushes back the movable contact shoe 8 to its closed position where the contacts touch again.
The present invention has been made in view of these points, and an object of the invention is to provide a structure of a current-interrupting section of a molded case circuit breaker, in which pressure springs of a movable contact shoe can easily be assembled in a contact shoe holder of a current-interrupting section.
Another object of the invention is to provide a structure as stated above, wherein a difference in corresponding spring forces between a contact activation position of the movable contact shoe and an open position by the electromagnetic reaction force resulting from over-current is minimized, so that it is possible to effectively prevent the spring from pushing back the movable contact shoe to the contact position, thereby improving the reliability of the opening operation.
Further objects and advantages will be apparent from the following description of the invention.
To attain the objects, the present invention provides a molded case circuit breaker having a current-interrupting section in a main body case of the circuit breaker. The current-interrupting section is composed of an assembly of an insulated case; an arc-extinguishing device; a power-supply side and load side fixed contact shoes diagonally arranged with respect to each other; a bridge-type rotary or movable contact shoe extending between contacts of the fixed contact shoes; and a rotary drum contact shoe holder for holding the movable contact shoe linked to a switching mechanism of the circuit breaker. The fixed contact shoes are formed in a U-shape so that an electromagnetic reaction force exerted by an over-current drives the movable contact shoe in an opening direction.
The movable contact shoe is loosely fitted in a hole formed on the contact shoe holder in a diameter direction, and pressed to be held in a position via a pair of pressure springs assembled inside the contact shoe holder. The pressure springs are torsion coil springs, and each spring has a U-shaped offset arm portion drawn out from a central portion of a coil. The torsion coil springs are arranged in upper and lower portions of the movable contact shoe. Legs of the torsion coil springs at ends are engaged with and locked to sidewalls of the contact shoe holder. Further, the offset arm portions engage the movable contact shoe at symmetrical positions with respect to a rotational center thereof.
Specifically, the circuit breaker can be constructed in the following configurations. The legs of each of the torsion coil springs, which are located at the respective ends thereof, are bent in an L-shape, and are engaged and locked by fitting the legs in holes formed in corresponding sidewalls of the contact shoe holder. Alternatively, the legs of each of the torsion coil springs, which are located at the respective ends thereof, are bent in an L-shaped, and are engaged and locked in engaging grooves cut out in peripheral edges of the corresponding sidewalls of the contact shoe holder.
Recess grooves are formed on top and bottom surfaces of the movable contact shoe at positions symmetrical with respect to the center of the movable contact shoe, and the offset arm portions of the torsion coil springs engage the corresponding recess grooves. Alternatively, stepped portions are formed on the top and bottom surfaces of the movable contact shoe piece at positions symmetrical with respect to the center of the movable contact shoe, and the offset arm portions of the torsion coil springs engage the corresponding stepped portions.
As described above, the pressure springs of the movable contact shoe are formed of a pair of the torsion coil springs, each having the offset arm portion drawn out from the center of the coil. The torsion coil springs are arranged in upper and lower portions of the movable contact shoe and interposed between the left and right sidewalls of the contact shoe holder. Thus, in comparison with the assembled structure in which the torsion coil springs are disposed at the right and left sides of the movable contact shoe, as in the conventional example, a larger coil length with a larger number of turns can be used to thereby minimize a spring constant. Then, when the movable contact shoe is driven in an opening direction by the electromagnetic reaction force resulting from the over-current, a quick opening operation is achieved. It is unlikely that the movable contact shoe will be pushed back to its closed position by the spring force to cause the contacts to touch each other once again. Consequently, this will result in more reliable circuit-breaking operations.
Furthermore, as compared to the conventional structure in which the pressure springs are the tension springs, the present invention does not require any assembly parts such as the rods on which the tension springs are hooked, thus simplifying the structure. Moreover, the movable contact shoe can be pressed and held in a position for assembly simply by pushing the springs into the contact shoe holder into which the movable contact shoe is fitted and inserted.